1. Field of the Invention
The present invention relates to methods of cleaning and rinsing silicon wafers or other wafers or thin disc-like substrates or element in the semiconductor process industry. The term "wafers" as used throughout this disclosure is meant to include any thin disc-like substrates in the semiconductor process industry, including substrates made of silicon, gallium arsenide, glass as well as masks made of polyamide material.
2. State of the Art
In the processing of wafers to make semiconductor devices, the wafers are subjected to numerous processing steps. Generally, the wafers must be cleaned and rinsed between each processing operation, and depending upon the particular process, the wafers may be put through a cleaner-rinser anywhere from 20 to 30 times. The condition of the wafers prior to each cleaning and rinsing depends upon the immediately preceding operation that the wafers have been through.
Cleaning and rinsing commonly serve two basic purposes. The first is to stop the etching process or other chemical process to which the wafers have been subjected. The second is to remove the etchant chemical or other chemical from the surface of the wafer. This has been accomplished by one of three commonly used methods: cascade overflow, tank dumping and spray spinning.
In the cascade overflow method, a tank is filled with deionized water until it overflows. The wafers are submerged in the upwardly flowing water in the tank. The upward flow of water carries chemicals and contaminants away from the wafers. Nitrogen is commonly bubbled into the upward flow of water to assist in cleaning and rinsing the wafers as well as reducing residual oxidation of the wafer surfaces.
Dump cleaning and rinsing has grown rapidly in popularity, and is a very effective method for accomplishing both of the aforementioned purposes of the clean and rinse operation. Using the dump method, the wafers are placed in a full cascading bath. Deionized water and nitrogen is admitted briefly as in the cascade method. The water is then rapidly dumped out the bottom of the tank through a trap door. The cycle is repeated several times.
In spin cleaning and rinsing, the apparatus operates on the principle of a centrifuge. Deionized water is sprayed on the wafers as they are spinning in the apparatus. Because of the spinning action, the wafers are subjected to high stress. In addition, wafer dust is generated and redeposited on the wafer surfaces along with other particulate accumulations.
There has been no suggestion of a method for cleaning and rinsing wafers wherein repeated cycles of sequences involving a cascade overflow step followed by a partial dump of the water from the tank. In the partial dump, the water level is not lowered below the top edges of the wafers in the tank, and the wafers are not therefor repeatedly subjected to air during the cleaning and rinsing operation. This greatly reduces inherent growth of oxide on the wafers as well as the migration of contaminants from the air to the wafers.
The parameters of the operation through which the wafers have passed immediately before cleaning and rinsing also effect the optimum parameters for the cleaning and rinsing operation. In particular, the amount of deionized water, the pH and other chemical content of the water as well as the temperature of the water are all dependent upon the conditions of the wafers as the wafers are received from previous operations. In the practices of the prior art, it has been common to disregard optimum conditions and instead use large excesses of deionized water during the cleaning and rinsing to overcome any inefficiency. It has also been common to use hot deionized water when it is really not needed. The hot deionized water can in fact be less effective than cool deionized water in many applications. However, there are certain applications when hot deionized water is useful. Deionized water, either hot or cold, is costly, and the wasteful practices of using excess water in the cleaning and rinsing operations is being discouraged, especially in arid areas where water is at a premium.
Various of the operations that the wafers undergo result in the surfaces of the wafers being hydrophobic. Some of the operations result in wafers having more hydrophilic surfaces. It has been recognized that wafers exhibiting hydrophobic surfaces are prone to interact with the cleaning and rinsing operation to yield widely varying particle counts of contaminants left on the wafers following the cleaning and rinsing. Wafers exhibiting more hydrophilic surfaces react much better to the cleaning and rinsing than the wafers having hydrophobic surfaces.
There has been no suggestion in the prior art of monitoring quality of the deionized water including contaminants contained in the water and chemical characteristics of the water in the tank and of using the quality monitoring to control the method of cleaning and rinsing. The quality of the water with respect to contaminants can be used, in accordance with the present invention, to control the cycling of steps of the method to achieve exceptional results in a minimum of time expended in the cleaning and rinsing operation. In addition, a minimum of deionized water is consumed. Monitoring of chemical characteristics of the deionized water allows the addition of chemicals to alter the chemical characteristics of the water to a more favorable condition, which again saves time and minimizes consumption of deionized water.
3. Objectives
A principal objective of the invention is to provide an improved method of cleaning and rinsing wafers that utilizes a combination of sequenced cascade overflow steps and partial dumps of the water in the tank, whereby the efficiency of the cleaning and rinsing is significantly improved so as to decrease time required in the cleaning and rinsing operation as well as to reduce the amount of deionized water used in the cleaning and rinsing. The partial dumping eliminates exposure of wafers to air and thus reduces the potential for oxide growth and particulate contamination on the wafers during the cleaning and rinsing operation.
A particular objective of the present invention is to provide a method wherein the deionized water in the cleaning and rinsing tank is monitored for a water quality characteristic indicative of contaminants in the water, with the number of cycles of sequenced cascade overflow steps and dump steps being controlled by the monitored characteristic to insure a rinse to a preset quality with a minimal amount of deionized water being consumed. The method can also be set to produce a warning indication when the preset quality has not been attained in a preset number of cycles of the sequenced cascade overflow steps and dump steps. Further, an enhanced cascade overflow mode can be activated at the same time or in place of the warning indication.
An additional objective of the present invention is to provide a sampling chamber connected to the cleaning and rinsing tank by a conduit having a valve therein, whereby the probe for monitoring of the water quality characteristic is contained in the sampling chamber and thus isolated from the cleaning and rinsing tank, and the valve is controlled to eliminate back flow of water from the sampling chamber to the cleaning and rinsing tank.
A still additional objective of the present invention is to provide a spray of fresh deionized water on the surface of the deionized water in the cleaning and rinsing tank at those times when water is overflowing the top of the tank so as to aid in removal of particulate contamination from the tank.
Another objective of the present invention is to provide a blanket of nitrogen over the top of the cleaning and rinsing tank to eliminate the necessity of a lid for the tank and to provide a nitrogen environment for the wafers during a full dump of the water from the tank as well as to eliminate particulate contaminants from migrating to the deionized water in the tank from the environment about the top of the tank.
Still another objective of the present invention is to provide for injection of nitrogen into the deionized water introduced into the cleaning and rinsing tank to aid in the cleaning and rinsing of the wafers and reduce normally inherent residual growth of oxide on the wafers in the tank.
Yet still another objective of the present invention is to provide a method wherein the deionized water in the cleaning and rinsing tank is monitored for a chemical characteristic of the water and/or the water-wafer system in the tank which is indicative of a chemical phenomena, whereby corrective chemicals can be added to the water in the tank to mitigate the chemical phenomena being monitored. For example, the pH and the oxidation reduction potential of the water in the tank can be controlled. Appropriate chemicals such as choline can be added to control the pH, and oxidizing agents such as hydrogen peroxide can be added when necessary to control the oxidation reduction potential. Control of the oxidation reduction potential mitigates the hydrophobic character of wafers and converts the wafers to a more hydrophilic character as well as to inhibit oxide growth on the wafers in the tank.
A further objective of the present invention is to provide a sampling chamber connected to the cleaning and rinsing tank by a conduit having a valve therein, whereby the probe for monitoring of the chemical characteristic of the water in the tank is contained in the sampling chamber and thus isolated from the cleaning and rinsing tank, and the valve is controlled to eliminate back flow of water from the sampling chamber to the cleaning and rinsing tank.
A still further objective of the present invention is to provide a hot deionized water manifold that allows the use of hot deionized water when certain chemicals are encountered in the cleaning and rinsing tank. The hot water acts to reduce surface tension adhesion characteristics of those chemicals as well as maintain a high solubility level for saturated buffered oxide etchants.
A yet still further objective of the present invention is to provide a method of cleaning and rinsing wafers that is always in a dynamic state, with a constant bleed of deionized water through the means for introducing the water to the cleaning and rinsing tank as well as through the sprayers at the top of the tank when such sprayers are employed. The constant bleed of deionized water inhibits bacterial growth in the apparatus of the method.
4. Related Application
This application is related to our copending application Ser. No. 07/561,778 for Apparatus for Cleaning and Rinsing Wafers which is being filed concurrently herewith and the entire contents of the copending application are incorporated herein by reference.